This invention relates to the recovery of oil from a subterranean reservoir through the use of surfactant flooding.
It has long been known that the primary recovery of oil from a subterranean formation leaves a substantial amount of the initial oil still in the formation. This has led to the use of what is commonly referred to as secondary recovery or waterflooding wherein a fluid such as brine is injected into a well to force the oil from the pores of the reservoir toward a recovery well. However, this technique also leaves substantial amounts of oil in the reservoir because of the inability of the water to wet the oil and the capillary retention of the oil. Accordingly, it has been suggested to use a surfactant in the waterflooding process. It has been found that the use of surfactants can reduce interfacial tension between the oil and the water to such an extent that substantial increased quantities of oil can be displaced.
Further efforts to better remove residual oil from subterranean deposits have focused on the use of microemulsions. In accordance with this technique, a microemulsion is prepared by mixing oil with brine and surface active agents. This technique, however, is inherently susceptible to many problems. For one thing, it is obviously undesirable to inject oil, which has already been recovered, back into the ground as is done in the conventional use of microemulsion. It is known to simply inject massive amounts of surface active agents into the ground to form a miscible microemulsion but this is not economical. Finally, the surfactant frequently precipitates, thus ending the effectiveness of the microemulsion. This latter phenomenon is frequently associated in the art with the presence of divalent ions such as calcium and magnesium, which are frequently unavoidably present in the formation. While it is possible to remove said divalent ions or excessively high concentrations of monovalent ions by means of preflush, this is undesirable because of the cost involved and should be avoided as much as possible.